Researchers tracking pregnant porbeagle sharks discovered the first evidence of the species being preyed upon by an even larger predator.
By
Just weeks before a pregnant porbeagle shark was expected to give birth, one of the two tracker tags attached by marine scientists surfaced near Bermuda.
The team hadn’t anticipated the tag appearing for months. They had affixed it to the 7-foot shark just 158 days earlier after capturing it off Cape Cod, Massachusetts, in October 2020 and performing an ultrasound. The pop-off tag was designed to remain attached for approximately a year.
“Something had gone very wrong,” said Brooke Anderson, then a shark researcher at Arizona State University.
The second tag, meant to transmit a signal when the shark’s fin breached the surface, would never send another signal.
The data from the recovered pop-off tag revealed an intriguing anomaly. For the first five months, the depth and temperature readings were typical for the species. Then, they went haywire.
“Suddenly, the temperature spiked, even at a depth of 600 meters, and remained elevated,” Anderson noted.
The shark’s diving patterns also became irregular.
“All the data led to the same conclusion: She had been consumed,” Anderson explained.
The researchers concluded that the tag's unusual readings were due to it being inside the stomach of another animal for several days.
Anderson and her colleagues detailed their findings in a study published Tuesday morning in *Frontiers in Marine Science*. This study provides the first evidence of a porbeagle being preyed upon by an even larger creature.
The study authors identified several potential predators and refined their list based on biological evidence. The tag’s temperature data ruled out warm-blooded mammals like orcas. Consequently, the researchers focused on endothermic sharks, which can regulate their body temperature to some extent.
“It had to be a shark capable of raising its body temperature above that of the surrounding water, large enough to inflict significant damage on the porbeagle, and residing in the area where the predation occurred,” Anderson explained.
The researchers deduced that either a white shark or a shortfin mako shark likely attacked the pregnant porbeagle and temporarily consumed the tag.
“I would speculate that it was a mature female white shark, probably over 15 feet in length,” Anderson noted.
This finding was surprising, as researchers previously doubted that porbeagle sharks could be preyed upon.
The team’s initial objective was to track pregnant porbeagle sharks throughout their pregnancy and determine their typical birthing locations.
Over the course of two seasons in the Atlantic, they successfully located and tagged 11 porbeagle sharks. Each shark was carefully lifted onto the boat, placed on deck, provided with a stream of aerated saltwater, and had its eyes covered with a wet towel.
“We work like a NASCAR pit crew,” said Anderson. Of the 11 sharks tagged, eight were pregnant.
Unexpectedly, the research led to the discovery of a deep-water predation mystery.
Matt Davis, a marine resource scientist with the Maine Department of Marine Resources who was not involved in the study, noted that the findings are “certainly within the realm of believability.” He emphasized that the case highlights how much more scientists need to learn about ocean life and predator-prey dynamics in deeper waters.
Porbeagle sharks are classified as vulnerable by the International Union for Conservation of Nature due to overfishing starting in the 1960s. By around 2001, the population had reportedly plummeted by 75% to 80%, according to Anderson.
Although fishing regulations have helped the species rebound, full recovery will take decades or more, given that porbeagles can live 30 to 40 years and reproduce relatively slowly compared to other species.
“We need to continue tagging and tracking these sharks to determine how frequently such predation events occur,” Anderson said. “In one instance, this already-vulnerable species lost not only a crucial reproductive female but also all her developing pups. It’s essential to understand the frequency and impact of such events on the population.”
Ultimately, their research could play a significant role in helping the species recover in the shark-eat-shark environment of the Atlantic.